There is a facial comparing technology for judging whether or not the persons represented by two images whose frontal facial images are captured are identical through image processing.
Usually, a facial comparison process begins with a normalizing process for uniformizing the positions and sizes of persons captured in respective two images in order to be able to compare the persons with each other. The facial comparison process then extracts facial features from the normalized images and compares the two images based on the extracted facial features. Existing facial comparison processes include PCA (Principal Component Analysis, Non-patent document 1) and LDA (Linear Discriminant Analysis, Non-patent document 2). The normalizing process for uniformizing the positions and sizes of faces captured in respective two images is very important for the facial comparison process according to the above processing sequence.
According to one normalizing method, the positions of left and right eyes are aligned by simply enlarging, reducing, and rotating images. According to another normalizing method, various parts of faces (hereinafter referred to as “facial parts”), such as left and right corners of eyes and mouths, and nostrils, are detected, and an affine transformation or a more complex linear transformation is carried out to minimize any positional misalignments of those facial parts.
The former normalizing method does not change aspect ratios and does not distort the shapes of the faces since the eyes are positionally aligned by simple enlargement, reduction, and rotation. The former normalizing method will hereinafter be referred to as “normalizing process with fixed aspect ratios”. For example, Non-patent document 4 employs a normalizing process using the positions of eyes.
The latter normalizing method changes aspect ratios and distorts the shapes of the faces while an affine transformation or a complex linear transformation is carried out. The latter normalizing method will hereinafter be referred to as “normalizing process with variable aspect ratios”. For example, in Non-patent document 3, a normalizing process is performed so as to make constant the positions of both eyes and the positions beneath the noses. Therefore, aspect ratios may be changed. Patent document 1 discloses a technology wherein the positions of feature points of faces in images to be compared are aligned with the positions of feature points of a standard facial image, after which the images are compared, and changes of the feature points are also used for facial comparison.
If the images of faces to be handled are captured obliquely, then frontal images recovered from the captured images can be used for a technology for comparing frontal facial images as described above (see Non-patent document 4).